This invention relates to a core element used for a fast breeder reactor, particularly to improvements on a pad construction of the core element.
A pad of a core element for a fast nuclear reactor will be explained hereinafter, taking an example of a nuclear fuel assembly.
Generally, the fuel assembly comprises a handling head of its upper portion, an entrance nozzle of its lower portion having orifices for coolant, and a wrapper tube of a right hexagonal tube protecting interior fuel pins and joining the handling head and the entrance nozzle. A reactor core is constructed by a plurality of the fuel assemblies arranged in honeycomb form with intervals of about 5 mm spaced therebetween. The fuel assemblies arranged in the honeycomb form each are provided with a pad so that the wrapper tubes of the fuel assemblies are spaced by the pads not to directly contact each other. Many pads used for conventional experimental nuclear reactor are formed by pressing out parts of the wrapper tube in round shape from its interior, that is, on each of six faces of the wrapper tube, a flat projection with a round periphery is formed.
On the other hand, it is known that the fuel assembly is bent by neutron beam gradient or temperature gradient, and interfere with other fuel assemblies at the pad portions of the fuel assembly. As nuclear reactors become large-scale, and have higher performance, load due to the interference increases, so that creep deformation of the pad and the wrapper tube will influence the interior fuel pins. Therefore, in order to keep the creep deformation small, various kinds of reinforced pads are proposed. For example in Japanese Pat. Publication No. 35677/1976, a cylindrical spacer pad (11) is disclosed. The spacer pad (11) is rigidly mounted on a hexagonal tube 1 by shrink fit. Practically, however, it seems to be difficult to rigidly mount the spacer pad (11) on the hexagonal tube (1) because tolerance of the tube 1, which is usually made by drawing, is relatively large. Further, the spacer pad 11 has a danger that the spacer pad (11) may fall off from the hexagonal tube 1 because axial ends of the spacer pad (11) of the hexagonal tube (1) may contact with other spacer pad (11) of other hexagonal tubes 1 when the hexagonal tube (1) is inserted in or removed from a core, and because deformation of the hexagonal tube (1) and the spacer pad (11), which deformation is caused by swelling of materials of the hexagonal tube (1) and the spacer pad (11) due to neutron radiation, may convert the rigid insertion of the hexagonal tube (1) to loose insertion. Another pad wherein a band is wound on a wrapper tube and fixed to the wrapper tube by welding, is known, for example in Japanese Pat. Publication No. 6439/1977. When welded, the pad is necessary to be formed to contact the wrapper tube. However, it is difficult to manufacture precisely the wrapper tube so that it will be necessary to reduce the pad or the wrapper tube to fit each other every fuel assembly, therefore much labor is required. Further, if heat supplied for welding is insufficient in order to decrease deformation of the wrapper tube due to the welding, strength of portions welded is insufficient, so that there is a danger that the pad may falls off from the wrapper tube. Even if the pad is welded sufficiently strongly, the welded pad has a danger that smooth insertion of the wrapper tube is prevented by the welded pad. Further welding accompanies a sufficient quality control and inspection, which results in increase of labor.
Accordingly, improved pad construction of core elements such as nuclear fuel assemblies, control rod assemblies and reflectors are desired.